As shown by F. N. Apel et al, U.S. Pat. Nos. 3,049,568, 3,049,569 and 3,153,001 and A. R. Grover et al, U.S. Pat. No. 3,221,061, ion-exchange resins have been found to be useful for effecting the condensation of acetone and phenol for making 2,2-bis-(4-hydroxyphenyl)propane of the formula, ##STR1## commonly referred to as "bisphenol-A" and referenced to hereinafter as the "4,4'-isomer". Experience has shown that, during the condensation reaction of acetone and phenol, in the presence of an ion-exchange catalyst, significant amounts of 2,4'-dihydroxy-2,2-diphenyl propane can be formed having the formula, ##STR2## referred to hereinafter as the 2,4-isomer.
When strong mineral acids, such as hydrochloric acid, are employed as catalysts in the condensation reaction, the 2,4-isomer can rearrange to the desired 4,4'-isomer under the reaction conditions. However, these reaction conditions result in the formation of significant amounts of tars which are difficult to salvage. These tars result in the formation of complex color forming aromatics which can adversely affect the utility of the desired 2,2-bis(4-hydroxyphenyl)propane monomer. As a result, efforts are constantly being sought to improve the effectiveness of ion-exchange catalysts for the formation of bisphenol-A in place of mineral acid catalyst, while minimizing the production of undesirable amounts of the 2,4'-dihydroxy-2,2-diphenyl propane, which can impair the purity of the desired bisphenol-A monomer.
One solution to the "isomer" problem when using an ion-exchange catalyst is to recycle the reaction by-products effluent from the bisphenol-A reaction stream after a concentration and recrystallization step. The by-product stream is then recycled to an isomerization zone to convert 2,4'-dihydroxy-2,2-diphenyl propane, hereinafter referred to as the "2,4'-isomer" to bisphenol-A, or the "4,4'-isomer" which can be further recycled to a feed tank along with makeup acetone and phenol and thereafter introducing the resulting isomerized mixture along with makeup feed to the reactor. Although in theory, the use of an isomerization step for the by-product feed which can be recycled to the reactor has achieved some degree of success as measured by improved rates of bisphenol-A production, the effectiveness of such isomerization procedure has often not achieved the degree of success desired. Improved isomerization procedures to convert the 2,4'-isomer to the 4,4'-isomer and accordingly more effective techniques to improve the rate of formation of bisphenol-A are constantly being sought.
As taught by Reed, U.S. Pat. No. 4,263,407, the term "macroreticular" as opposed to "microreticular" means porous adsorbance in which the pores are larger than atomic distances and are not part of the polymer structure per se. Rather, the pores are microscopic channels resulting from the squeezing out of an organic precipitant from a copolymer mass. As a consequence, the pore structure is not dependent upon environment and therefore is retained despite contact with various concentrations of electrolyte, solvent and exchangable ions.
In microreticular resins (gel-type) the pores are not really pores at all because they are extremely small, usually below 30 A in diameter and will disappear from the polymer structure when the polymer is dry. The microreticular gel resin has a continuous polymer phase while the macroreticular resin is clearly shown to consist of conglomerates of granularly packed macrospheres with both a continuous polymer phase and a continuous void phase. Thus the expression "porous" as used herein refers to channels or openings between conglomerates of minute spherical particles.
The present invention is based on the discovery that macroreticular cation-exchange catalyst have been found to be particularly useful for directly converting the 2,4'-isomer to the 4,4'-isomer. Accordingly, an improved procedure for isomerizing the 2,4'-isomer to the 4,4'-isomer has now been found.